1. Field of the Invention
This invention relates to a semiconductor integrated circuit device and a method of operating same, and more particularly to a semiconductor integrated circuit device including memory cells for storing multilevel data and a method of operating same.
2. Background Art
In recent years, nonvolatile semiconductor memory devices, which allow electric bulk erase and rewrite of data and in which the written data can be held without power supply, are widely used particularly in mobile devices. Such a nonvolatile semiconductor memory device is composed of memory MOS (Metal Oxide Semiconductor) transistors, which each have a tiny floating gate electrode surrounded by an insulating film, and interconnects for data input/output. The memory device retains memory by accumulating electric charge in the floating gate electrode.
Recently, nonvolatile semiconductor memory devices are downscaled, and the spacing between adjacent memory cells is significantly narrowed. As the cell-to-cell spacing is narrowed, the capacitance between floating gates in adjacent cells increases. Then the threshold of the previously written cell tends to vary because it is more susceptible to the capacitance between floating gate electrodes with its adjacent cell that is subsequently written. In particular, a multilevel memory (JP 2004-192789A), which stores multilevel data in one cell, has a plurality of thresholds. Hence the threshold distribution per one data must be controlled in a very narrow range, and unfortunately, the threshold is likely to vary in response to the accumulated charge in its adjacent cell.